1. Field of the Invention
The present invention relates to an image sensor and a fabricating method thereof. More particularly, the present invention relates to a complementary metal-oxide-semiconductor image sensor (CIS) and a fabricating method thereof.
2. Description of the Prior Art
The fabrication process of a complementary metal-oxide-semiconductor image sensor (CIS) is compatible with that of a CMOS, so it is easy to be integrated with other peripheral circuits on the same chip, thereby greatly reducing the cost and power consumption of the image sensor. In recent years, in the field of low price applications, CIS becomes a substitute for charge-coupled devices, and thus the importance of CIS is increased rapidly.
A CIS is constituted by a photo diode (PD) and a plurality of transistors. The PD is constituted by a p-n junction formed by an n-doped region and a p-type substrate, and the transistor is an n-type transistor with an n-type gate (n-poly NMOS). At present, the structure of a CIS may be classified into two categories, namely the 3-T architecture and the 4-T architecture. The 3-T architecture refers to the structure of the CIS includes a reset transistor (Rx), a source follower transistor (Dx), a selection transistor (Sx), and a photo diode (PD). The 4-T architecture refers to the structure of the CIS includes a transfer transistor (Tx), a reset transistor, a source follower transistor, a selection transistor, and a PD.
Currently, it is a common problem that a dark current may be generated in the CIS. Generally speaking, during the fabrication process of the CIS, before a gate oxide layer is formed, an implantation process is performed to implant a dopant into the substrate at the position for forming a gate of a transfer transistor. Or, another implantation process is performed to implant a dopant into the substrate on periphery of an isolation structure adjoining to a photo-sensitive region, thereby forming a protection layer in the implantation region to alleviate the dark current.
However, when a thick gate oxide layer or a composite structure of conductive layer/dielectric layer/conductive layer needs to be formed in the fabrication process of the CIS, a severe diffusion of the protection layer may occur due to a thermal process, so the effect on eliminating the dark current is limited, and also the pixel performance of the image sensor may be adversely affected. Moreover, in addition to the above problem, the ion implantation process performed before forming the gate oxide layer may damage the surface of the substrate during the ion implantation process.